1. Field of the Invention
The invention relates to an image forming apparatus such as copying machines, printers and facsimile machines, and more particularly to an image forming apparatus of an intermediate transfer type for transferring an image formed on an image forming carrier to a recording medium via an intermediate transfer medium.
2. Description of the Related Art
There is a known image forming apparatus (e.g., JP-A-2-212870) as a related-art image forming apparatus of the intermediate transfer type, which has been provided with developing units for yellow (Y), magenta (M), cyan (C) and black (B) color components, for example, around an image forming carrier such as a photosensitive drum and an intermediate transfer medium in the form of a belt (intermediate transfer belt), for example, disposed opposite to the image forming carrier. The conventional image forming apparatus above is used for forming a desired image on paper by making the primary transfer of unfixed toner images with various color components formed on the image forming carrier per rotation of the image forming carrier sequentially to the intermediate transfer belt and also makes the secondary transfer of a composite primarily-transferred image laid on top of each other on the intermediate transfer belt on the paper as a recording medium.
In this case, a secondary transfer device is employed, having, for example, a transfer roll so disposed as to be in contact with the intermediate transfer belt, a backup roll disposed opposite to the transfer roll with the intermediate transfer belt held therebetween, and a power supply for applying bias between the transfer roll and the backup roll for forming an electric field causing the toner images on the intermediate transfer belt to be transferred on the paper.
As the composite primarily-transferred image that has already been subjected to multiplex transfer on the intermediate transfer belt is collectively transferred on the paper, the image forming apparatus of the intermediate transfer type is advantageous in that unstable factors at the time of forming images become removable and that generation of image disorders and color drift can effectively be prevented at the time of the multiplex transfer.
In the above image forming apparatus of the intermediate transfer type, however, since the secondary transfer device provided separately from a primary transfer device is indispensable, a relatively high-cost functional member (transfer roll) has to be disposed on the periphery of the intermediate transfer medium, which tends to increase not only space for the exclusive use of the secondary transfer device but also the number of parts, thus resulting in increasing the size of the image forming apparatus itself as well as costs.